1. Field of the Invention
The present invention relates to a power source voltage protective device and a power source voltage protection method for detecting a state of a power source voltage that is supplied to a CPU, a microcomputer, or the like.
2. Related Art
Abnormal reduction or increase of a power source voltage that is supplied to a CPU, a microcomputer, or the like may cause trouble in operation of the CPU, the microcomputer, or the like. In view of this, conventionally, methods for detecting a state of a power source voltage supplied to such a device have been proposed (refer to Patent documents 1 and 2, for example).    Patent document 1: JP-A-2009-239769    Patent document 2: JP-A-11-103522
In the device disclosed in Patent document 1, a power source voltage and a reference voltage are input to an AD conversion device and reduction of the power source voltage is detected by comparing data obtained by AD-converting the reference voltage using the power source voltage and data obtained by AD-converting the power source voltage using the reference voltage. The power circuit disclosed in Patent document 2 performs protection processing of detecting reduction of a power source voltage by comparing a reference voltage obtained by multiplying the power source voltage by a prescribed coefficient with a voltage that is separately input from outside.
In the above-described methods, in order to detect a state of a power source voltage, a voltage other than the power source voltage is compared with the power source voltage. For example, in the device of Patent document 1, AD-converted data of a power source voltage and a reference voltage are compared with each other. In the circuit of Patent document 2, a reference voltage and a separately input voltage are compared with each other (voltage values are compared with each other). However, in configurations in which a power source voltage and a comparison subject voltage are compared with each other, either the power source voltage being higher than the comparison subject voltage or the power source voltage being lower than the comparison subject voltage can only be judged. For example, the configurations of Patent documents 1 and 2 enable detection of an abnormality that the power source voltage is too low, they are unable to detect an abnormality that the power source voltage is too high. To detect both of such abnormalities by the conventional methods, it is necessary to use many comparison subject voltages. That is, more voltages of different systems need to be input to an AD conversion device, a microcomputer, or the like, which causes problems such as complication of a circuit configuration and resulting cost increase.